Light communication is a wireless communication technology that uses light to accomplish communication, in which the light may be sensible by one or more photo diodes, or an image sensor. For example, the light may be, but not limited to, x-rays, ultra-violet light, visible light, or infrared with frequency being in the range from 109 Hz to 1019 Hz. RF (radio waves) bandwidth is a scarce resource. Therefore, light communication may provide an alternate technology to meet the strong demands of wireless communications. For example, the visible light emitted from one or more light emitting diodes (LEDs) is widely used in homes and offices, thus it makes the visible light emitted from the one or more LEDs ideal for ubiquitous data transmitters. Cameras are pervasive on phones, laptops and many handheld electronic devices. Thus an image sensor may be used not only as an incoming image detector, but also as a receiver in a visible light communication (VLC) system. FIG. 1 shows a schematic view of an exemplary system model for light communications using an LED array 112 and an image sensor such as a high-speed camera 122. As shown in FIG. 1, an encoder 114 in a transmitter 110 may perform the physical layer processing, such as error-correction encoding, modulation, OFDM, etc. A decoder 124 in a receiver 120 may perform the inverse signal processing of the encoder 114. FIG. 2 shows a schematic view of another exemplary another system model for light communications using a single LED 212 and an image sensor such as a high-speed camera 222.
An image sensor may be composed of an array of photosensites. CCD and CMOS are two basic types of digital sensors. For a CCD sensor, the first row of the array of photosensites is read into an output register, which in turn is fed into an amplifier and an analog to digital converter (ADC). After the first row has been read, it is dumped from the read out register and the next row of the array is read into the register. For a CMOS sensor, each photosite in the CMOS sensor has three or more transistors. The transistors allow for processing to be done right at the photosite, and each pixel/photosite can be accessed independently. For any one of the two types of image sensors, only one or few shared ADCs are used for all pixels in the sensed image in some existing light communication techniques.
FIG. 3 shows a schematic view of a technique for visible light communication. In FIG. 3, the pixels in the non-interested region of the sensed image are also sampled by the shared ADC(s) of image sensors in a receiver 320, and then discarded by an Rx baseband processing unit 326 in the receiver 320. In other words, this technique takes all the photographed pixels from the image sensor 322, and uses a luminance change detection to detect the region of interest (ROI). Taking all pixels in an image may limit the frame rate up to several hundreds of frames per second. However, the data rate requirement for wireless communications usually has an order of 10 M to 1 G bits per second.
FIG. 4 shows a schematic view of another technique for visible light communication. In the visible light communication system of FIG. 4, a transmitter 400 has a modulation circuit 410 to generate different modulated signals 412, and a light-emitting drive circuit 420 to drive a light-emitting element 422 according to the modulated signals 412 output from the modulation circuit 410 and used as a drive signal 421. While the drive signal 421 remains at a high level, the light-emitting drive circuit 420 makes the light-emitting element 422 emit visible light 440 that is superposed with the signal 402. While the drive signal remains at a low level, the light-emitting drive circuit 420 makes the light-emitting element 422 emit no light.
Light communication is still a potential solution to the global wireless spectrum shortage. Various solutions for visible light communication techniques have been suggested. In general, the light-emitting devices such as LED(s), and the image sensor(s) such as camera, are existed devices in these solutions for visible light communication. However, the LED has low bandwidth in the frequency response, and the image sensor has low frame rate due to the shared ADC(s). There are challenges in these solutions for high data rate transmission of visible light communication.